In the field of electrophotography, there are generally two types of copiers, coated paper copiers and plain paper copiers. Coated paper copiers generally have a photoconductive material coated onto a paper substrate. The coated paper is charged, imaged and developed to produce a copy. There are generally two ways of charging the coated paper, either through a corona or through charge rollers. In the charging by use of charge rollers, the coated paper is conveyed between two biased rollers, one of which is resilient and the other rigid.
Prior charge roller systems for electrophotographic copiers do not impart uniform charging to a photoconductor over the entire range of ambient conditions. The main reason for this shortcoming is that the resistance of the charge rollers can change significantly with variation in the temperature. This change in resistance prevents the charge roller from working at low temperatures because the voltage at the paper charge roller interface is not great enough to charge the copy paper. Decreasing the thickness of the conductive rubber on the roller would decrease the variability in resistance and give more stable operation over the environmental envelope, except that new problems arise due to the thinner cover. The reduced coating thickness causes high power supply current to be drawn, due to the reduced impedance, and possibly could cause corona due to the sharp edge of the metal insert. This is especially true where the negative charge roller comes in contact with a positive metal ground roller and where the paper does not extend to the end of the conductive rubber cover.